Abstract
The enthalpically favoured hydration of hydrophobic entities, termed hydrophobic hydration, impacts the phase behaviour of numerous amphiphiles in water. Here, we show experimental evidence that hydrophobic hydration is strongly determined by the mean energetics of the aqueous medium. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. This denotes the energetics of water as a key parameter controlling the phase behaviour of PNiPAM and identifies the excess mixing enthalpy of water/kosmotrope mixtures as a gauge of the kosmotropic effect on hydrophobic assemblies.
Highlights
Correspondence and requests for materials should be addressed to Hydrophobic hydration of poly-N-isopropyl acrylamide: a matter of the mean energetic state of water
We show that when hydrophobic hydration is the prevailing contribution controlling the phase behaviour of an amphiphilic polymer, such as poly-N-isopropyl acrylamide (PNiPAM), the impact of a given kosmotrope on the phase behaviour is directly correlated to the energetics of the solvent; thermodynamic characteristics defining the phase transition of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/kosmotrope mixtures becomes minimal
The boundary between the two-phase and one-phase regime observed at higher X is characterized by an upper critical solution temperature (UCST) that depends on both molecular weight and concentration of PNiPAM; as an example we show in Fig. 1 the UCST boundary obtained for a PNiPAM system with a viscosity averaged molecular weight of Mv 5 39 000 g/mol and a concentration of c 5 1022 g/ml
Summary
Correspondence and requests for materials should be addressed to Hydrophobic hydration of poly-N-isopropyl acrylamide: a matter of the mean energetic state of water. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. Regarding the tunability of hydrophobic assemblies this correlation provides a valuable guideline on how to predict the impact of the kosmotropic effect when using different types of kosmotropic organic solutes, without the need of developing a full thermodynamic model, which can be very complex in such multi-component systems
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